Electric Power Steering (EPS) systems require the electric motor used to provide steering assist to be operated using a method of torque control. When using a Permanent Magnet Synchronous Machine (PMSM), Field Oriented Control (FOC) may be utilized. FOC transforms the AC phase motor voltage and current signals in the stationary reference frame to the synchronously rotating reference frame, commonly referred to as the d/q axis reference frame, in which the motor voltages and currents become direct current (DC) quantities. FOC torque control is commonly implemented through a closed loop current control method that employs current regulators to minimize the error between commanded and measured currents to achieve perfect current tracking. Thus, current control requires the motor currents to be measured, which may be achieved by measuring the phase currents of the electric machine, which are then transformed into the synchronous frame via the Park Transform to perform the control in the synchronous reference frame.
When an offset error of a given magnitude occurs in a phase current measurement, the closed loop current control operating in the synchronous reference frame adjusts the motor voltage so that the measurement of motor current matches the command. Since the measurement is incorrect, the actual motor currents are also incorrect. The failure mode results in motor position dependent motor torque and current errors, which may be perceived as a large torque ripple at the motor shaft, and potentially larger than rated motor currents (for the hardware design). When the torque ripple caused by the phase current measurement offset error exceeds a certain threshold, the offset error can produce motor torque in the opposite direction from the motor torque command. When used in EPS systems, failures which produce torque in a direction opposite to the desired motor torque command result in efforts above manual from the driver.